An international team of astronomers using data from several NASA and European Space Agency (ESA) space observatories has discovered unexpected behavior from the supermassive black hole at the heart of the galaxy NGC 5548. Their findings may provide new insights into the interactions of supermassive black holes and their host galaxies.
Researchers detected a clumpy gas stream flowing rapidly outward from a supermassive black hole, blocking 90 percent of its emitted X-rays. This deep look into a black hole's environment yields clues to the behavior of black holes embedded in the center of active galaxies.
"There are other galaxies that show gas streams near a black hole, but they haven't changed their position as dramatically," said Gerard Kriss of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. "This is the first time we've seen a stream like this move into the line of sight. We just happened to get lucky."
According to Kriss, the streamer is long-lived, and just recently started crossing Hubble's line of sight. In galaxies such as NGC 5548, streamers at an inclination this high above the accretion disk are rare. This overhead view of the black hole and its environment provides a new perspective in the structures of outflowing material associated with active black holes.
The discovery was made during an intensive observing campaign with NASA's Hubble Space Telescope, Swift spacecraft, Nuclear Spectroscopic Telescope Array (NuSTAR), and Chandra X-ray Observatory, and ESA's X-ray Multi-Mirror Mission (XMM-Newton) and Integral gamma-ray observatory. The international team, led by scientist Jelle Kaastra of the SRON Netherlands Institute for Space Research, conducted the most extensive monitoring campaign ever of an active galaxy in 2013 and 2014.
This is the first direct evidence of a long-predicted shielding process that is needed to accelerate powerful gas streams, or winds, to high speeds. These winds only occur if their starting point is shielded from X-rays. The winds may be so strong that they can blow off gas that otherwise would have fallen onto the black hole. Black hole winds can therefore regulate both the growth of the black hole and its galaxy.
The newly discovered gas stream in the Type 1 Seyfert galaxy NGC 5548 – one of the best-studied sources of this type over the past half-century – provides this protection. It appears the shielding has been going on for at least three years.
Immediately after Hubble observed NGC 5548 on June 22, 2013, the team discovered unexpected features in the data.
"There were dramatic changes since the last observation with Hubble in 2011. I saw signatures of much colder gas than were present before, indicating that the wind had cooled down due to a strong decrease of ionizing X-ray radiation from the nucleus," Kriss said.
After combining and analyzing data from the six observatories, the team was able to put the pieces of the puzzle together. Supermassive black holes in the nuclei of active galaxies, such as NGC 5548, expel large amounts of matter through powerful winds of ionized gas. For instance, the persistent wind of NGC 5548 reaches velocities exceeding 621 miles (approximately 1,000 kilometers) per second. But now a new wind has arisen, much stronger and faster than the persistent wind.
"The new wind reaches speeds of up to 3,107 miles (5,000 kilometers) per second but is much closer to the nucleus than the persistent wind," Kaastra said. "The new gas outflow blocks 90 percent of the low-energy X-rays that come from very close to the black hole, and it obscures up to a third of the region that emits the ultraviolet radiation at a few light-days' distance from the black hole."
